The present invention relates generally to instruments for measuring transmission of optical components, and more particularly to a high resolution infrared (IR) densitometer for measuring optical densities up to about 6 in the 1-5 micron (.mu.) spectral range.
The densitometer described herein meets the extremely exacting need for measurement of optical attenuation across a narrow band of an IR signal within the 1-5 .mu. wavelength range. The invention has a specific application in the characterization of high density rejection line filters, for which a high resolution system is required. Attenuation levels encountered in such an application greatly exceed measurement capabilities of conventional equipment (typically limited to densities of 3-4 over a 2-3% bandwidth). Measurement of a large attenuation factor requires a system with a correspondingly high signal-to-noise ratio. Thus to measure a density of 6, the optical power to be delivered to the detector must be 10.sup.6 times the noise level of the detector; the light source must have high power output and the detector must have a low noise level. Since the system must operate at any wavelength across a broadband, both light source and detector must be broadband (in contrast to conventional equipment which operate on fixed wavelengths), and the system should be capable of characterizing notch filters.
The only practical light source continuously covering the 1-3 .mu. region is an incandescent source, or hot filament. A thermal source of this type, while providing requisite power across the band, operates only with simultaneous output at all wavelengths. The broadband detector responds to each of these wavelengths. The intense broadband radiation at wavelengths other than the very narrow one at which a measurement is made must be removed or it will contribute a noise signal which swamps the measurement. Typically, the stray light needs to be reduced by a factor of about 10.sup.9 while the signal at a selected wavelength is preserved, which calls for a passband filter to be placed in series with the stop band notch filters. Thus the system filter must have a continuously tunable passband wavelength and must reject stray light by a factor of about 10.sup.9. A useful filter means for the system is a high-performance grating spectrometer. The densitometer of the invention therefore comprises three basic components, viz, an intense thermal light source, a low-noise broadband detector and a spectrometer for stray light rejection.
It is therefore a principal object of the invention to provide an improved densitometer.
It is a further object of the invention to provide a high resolution infrared densitometer for measuring optical densities to about 6 in the 1-5 .mu. spectral range.
It is another object of the invention to provide an infrared densitometer for measuring optical densities of high density optical filters.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.